思路： 简单回溯。

class Solution {public:    vector
    
     
      > combinationSum(vector
      
       & candidates, int target) {        vector
       
        
          > res;         vector
         
           combination; sort(candidates.begin(), candidates.end()); DFS(candidates, target, res, combination, 0); return res; } void DFS(vector
          
           & candidates, int target, vector
           
            
              > &res, vector
             
               &combination, int step) { if (target == 0) { res.push_back(combination); return; } for(int i = step; i < candidates.size() && target >= candidates[i]; i++) { combination.push_back(candidates[i]); DFS(candidates, target-candidates[i], res, combination, i); combination.pop_back(); } }};